Stabilized zirconium oxide for observation window

ABSTRACT

In order to produce an observation window ( 7 ) in reaction vessels, autoclaves, boilers, nuclear reactors and the like, a cubically stabilized zirconium oxide monocrystal can be used. Preferably, calcium, magnesium, yttrium or another transparent element of the heavy lanthanoids is used to stabilize zirconium oxide. Due to the excellent transparency of such observation windows ( 7 ) produced by means of a cubically stabilized zirconium oxide moncrystal, they are especially suitable for monitoring processes in situ in closed vessels such as the above-mentioned reaction vessels and the like, the processes being monitored either optically by means of photographic appliances or video appliances ( 1 ), or by means of spectroscopic methods.

[0001] The present invention relates to the used of stabilized zirconium oxide monocrystal for windows used in chemical reactors, autoclaves, boilers, nuclear reactors, particularly boiling-water reactors, etc., i.e. in closed containers wherein processes are conducted under extreme conditions such as elevated temperature, high pressure, acid or basic (oxidizing or reducing) environments, etc. In addition, the present invention relates to observation windows for chemical reactors, autoclaves, boilers, nuclear reactors, etc. as well as monitoring, control, and analysis processes using an observation window according to the invention.

[0002] For in situ monitoring of reaction processes in reaction vessels, such as in particular autoclaves, boiling-water or pressurized-water reactors, and the like, observation windows are provided whose window material is chosen according to the requirements. In measurement and monitoring of classical reactions and reactions taking place under extreme conditions, i.e. below or above thermodynamically critical conditions of fluids, or corrosive solutions (acid or basic—reducing or oxidizing—conditions), to determine the progress of reactions or of structural changes in industrial, chemical, or nuclear facilities, increased requirements are imposed on the material chosen for the observation window. Ordinary glass may suffice in reactors at room temperature or only slightly elevated temperatures, or non-aggressive atmospheric conditions, when perfect transparency of such an observation window for a relatively short period of time is required for monitoring the reaction process. In cases where the reaction process is to be monitored by spectroscopic methods for example, ordinary glass such as quartz or sapphire is often not the appropriate material. In cases where processes such as reaction processes are conducted at high temperatures, high pressures, and under aggressive atmospheric conditions, and process control or monitoring with the aid of optical or other analytical methods is required, a suitable material must be chosen that meets the requirements for transparency.

[0003] Thus, for example, under boiling-water-reactor conditions for monitoring corrosion in the stainless steel used for the reactor cladding such as a zirconia alloy (Zircalloy) by diffusion reflection spectroscopy (DRS), it has been proposed that an observation window made of sapphire glass be used, i.e. consisting of α-aluminum oxide (synthetic sapphire). This material remains transparent over a long period of time even at high temperatures and is commercially available at an acceptable price.

[0004] The use of sapphire windows for monitoring reaction phenomena in particular has been described in a number of patents, such as two U.S. Patents U.S. Pat. No. 4,666,251 and U.S. Pat. No. 5,709,471 and two Japanese Patents JP 4,001,709,A2 and JP 6,318,647 A2.

[0005] Thus, it has been shown that sapphire windows remain transparent even under boiling-water conditions, but lose their transparency after for example 20 days of contact with hot water.

[0006] It has also been found that elevated chemical aggressiveness, for example under acid or basic conditions—in a reducing or oxidizing environment—and at temperatures even higher than the boiling point of water, sapphire has a slight degree of surface solubility which reduces its transparency.

[0007] The goal of the present invention is therefore to find a material for observation windows of the type described that remains transparent at high temperatures and under the above-mentioned extreme environmental conditions.

[0008] According to the invention, the use of zirconium oxide (zirconia) or stabilized zirconium oxide or zirconia as described in claim 1 is proposed.

[0009] It has been shown that, by using stabilized zirconium oxide monocrystal or zirconia, observation windows in reaction vessels, autoclaves, boilers, and nuclear reactors, particularly boiling-water reactors, can be made which, even at high temperatures, high pressure, and/or under extreme conditions such as acid or basic (oxidizing or reducing) environments remain transparent for a prolonged period of time.

[0010] The use of zirconia for other applications is known of itself. For example, in U.S. Pat. No. 5,036,767, cubic zirconia for optical windows of laser-injected explosive devices is proposed. In U.S. Pat. No. 5,949,536, the use of cubic zirconia for optical cells in conjunction with spectroscopic analysis instruments is proposed. This patent proposes other materials that are entirely inappropriate for use in observation windows in reaction vessels, as proposed in the present invention. U.S. Pat. No. 5,769,540 again proposes a zirconia coating on a substrate in conjunction with optical methods for measuring surface properties. U.S. Pat. No. 5,490,728 deals with the same area of application. However, in none of these patents is zirconia or stabilized zirconium oxide monocrystal mentioned or suggested in connection with the use of observation windows in reaction vessels, autoclaves, boilers, and the like as described in the present invention.

[0011] Preferably, a zirconium oxide material stabilized with calcium, magnesium, or yttrium is used for making the aforesaid observation windows.

[0012] The purity of the stabilized zirconia must be better than 98%.

[0013] The proportion of chromatic impurities should be as low as possible and depends on the type and use of the observation window.

[0014] The surface roughness of the window made using stabilized zirconia must be as small as possible and is normally in the nanometer range, once again depending on the application. For example, the surface roughness of such an observation window is approximately 1 nm on average and in no case should be less than 3 nm.

[0015] A preferred material for making the aforesaid observation window is yttrium-, calcium-, or magnesium-stabilized zirconium oxide or zirconia, with the proportion of cubic monocrystalline material being approximately 90 mol % zirconium oxide. Instead of yttrium, another transparent element in the (heavy) lanthanoid series may be used.

[0016] The example below of an yttrium-cubic-stabilized zirconium oxide window will serve for better understanding of the present invention: Parameter Value/Unit formula Y_(0.15)Zr_(0.85)O_(1.925) structure monocrystalline purity >98% transparency 300* to 2000 nm (or better) density 6.0 g/cm³ hardness 1100-1500 kg/mm² pulse strength 69-105 kg/mm² specific heat capacity 460 J/kg/K heat conductance ˜2.0 W/m/K thermal expansion 10.3 × 10⁻⁶/K refraction index n 2.10 dn/dT 7.9 × 10⁻⁶/K acid resistance extremely high solubility at pH 1 2 × 10⁻⁵ M hydrolytic resistance Inert water solubility ˜10⁻¹⁰ M corrosion rate in twice- 90 ng/cm²/h 1) distilled water at 300° C. basic resistance extremely high solubility at pH 13 2 × 10⁻⁵ M

[0017] For monitoring reaction processes for example or the above-mentioned corrosion in boiling-water reactors, a single window made of stabilized zirconium oxide monocrystal or zirconia may be used, if the analytical method used for monitoring is operated in the so-called reflective mode. This means that the beam passing through the window and entering the medium to be monitored is reflected at the wall of the process container opposite the window and exits through the same window made of optical stabilized zirconium oxide or zirconia for analytical evaluation.

[0018] Based on the accompanying FIGURE, the use according to the invention of a zirconium oxide window for monitoring a process according to the following invention will be described in greater detail.

[0019]FIG. 1 shows an experimental arrangement according to the present invention schematically, in section. The diagram shows an experimental arrangement fitted with an optical stabilized zirconium oxide window for process monitoring in the so-called reflective mode.

[0020] A measuring cell or measuring sensor 1 is provided, such as an optical fiber system, connected with spectroscopic or optical measuring sources such as a spectrometer or a video camera as well as with an evaluation unit. The measuring cell is embedded in a metal container such as stainless steel or an inert ceramic.

[0021] The signal emitted by the measuring cell first passes through an air chamber 5 then through the optical stabilized zirconium oxide window 7 proposed according to the invention. To the rear of the zirconium oxide window is the space containing the fluid 9 to be investigated or analytically monitored for example, which may be for example hot water or a corrosive or reactive solution.

[0022] It is essential for analytical monitoring that the signal emitted by the measuring sensor be reflected; this occurs by means of the surface 11 which is as completely reflective as possible.

[0023] Since it is advantageous or merely necessary for only a portion of the fluid being monitored to be analyzed on-line, most of the investigated fluid 13 moves along the rear side of the reflecting surface 11. Finally, the measuring arrangement is closed off by a rear wall 15.

[0024] Monitoring of a process is of course not confined to the use of only one observation window made of stabilized zirconium oxide, but it is also possible to use such observation windows in series for monitoring by passing the beam used for monitoring by transmission through the aforementioned unit such as a reactor, autoclave, boiling-water or pressurized-water reactor, water heater, boiler, etc.

[0025] The optical stabilized zirconium oxide window can for example be sealed from the reactor wall or autoclave wall, etc. by Teflon, carbon graphite, rubber, or a soft metal gasket, or be pressed against the container wall. The seal may also be as proposed in U.S. Pat. No. 5,046,854.

[0026] Because of the high strength and inertness of the optical stabilized zirconia window, it may be used for monitoring water, acid or basic solutions, or organic solutions, or for monitoring processes conducted at high temperatures, for example in the region of 200 to 1000 Kelvin, and in processes under pressure in a range of e.g. 1 to at least 500 atm.

[0027] The optical transparency remains in a spectral range of at least 300-2000 nm, or is preferably constant, particularly in contact with transparent fluids and high temperatures well over 500 Kelvin, and when neutron or gamma radiation is used.

[0028] As the corrosion comparison of stabilized zirconium oxide, sapphire, and glass in the above example under the same conditions shows, the corrosion of stabilized zirconium oxide is considerably less than that of sapphire and glass. This lower corrosiveness comes from the considerably lower surface solubility of cubic stabilized zirconium oxide, compared with the solubility of sapphire or glass.

[0029] Stabilized zirconia windows may be used for monitoring for weeks, months, or even years; of course the useful life depends on the system to be monitored and the window transparency requirements.

[0030] An observation window made of stabilized zirconium oxide of the type defined according to the invention may be used for optical monitoring using photographic apparatus, video monitoring, and spectroscopic monitoring in the UV, visual, or infrared ranges.

[0031] The dimensions of the zirconium oxide window used are of course governed by the requirements, particularly the mechanical demands, of the process or reaction conditions prevailing in the reaction vessel, autoclave, etc.

[0032] Finally, it should be pointed out that the cost of stabilized zirconium oxide as proposed according to the invention is no higher than the cost of sapphire. On the contrary, stabilized zirconium oxide is less costly than diamond for example. The company Mateck-Material-Technologie und Kristalle GmbH, Dr. H. Schlich, D-52428 Jülich, may be mentioned as representative of a number of vendors. 

1. Use of stabilized zirconium oxide monocrystal for observation windows in reaction vessels, autoclaves, boilers, nuclear reactors such as boiling-water reactors, and the like.
 2. Use according to claim 1, characterized in that zirconium oxide stabilized with calcium, magnesium, or yttrium is used.
 3. Use according to one of claims 1 or 2, characterized in that the purity of the cubic stabilized zirconium oxide is at least 98%, preferably >99%.
 4. Use according to one of claims 1 to 3, characterized in that the cubic stabilized zirconium oxide or zirconia is stabilized by yttrium or another transparent element in the heavy lanthanoid series.
 5. Use according to one of claims 1 or 4, characterized in that the stabilized zirconia monocrystal contains approximately 90 mol % zirconium dioxide.
 6. Use according to one of claims 1 to 5, characterized in that the stabilized zirconium oxide or zirconia has the following formula: Y_(0.15)Zr_(0.85)O_(1.925).
 7. Observation window in reaction vessels, autoclaves, boilers, nuclear reactors, particularly boiling-water or pressurized-water reactors, and the like, characterized in that the window consists at least largely of zirconium oxide or zirconia, particularly of stabilized zirconium oxide monocrystal.
 8. Observation window, particularly according to claim 7, characterized in that the cubic stabilized zirconium oxide window is sealed with PTFE (Teflon), carbon graphite, rubber, or a soft metal gasket inserted into the container wall.
 9. Observation window according to one of claims 7 or 8, characterized in that zirconium oxide stabilized with calcium, magnesium, yttrium, or another transparent element in the heavy lanthanoid series is used as the stabilized zirconium oxide monocrystal.
 10. Use of the observation window according to one of claims 7 to 9 for in situ process monitoring in closed vessels, particularly in reaction vessels, autoclaves, boilers, boiling water reactors, and the like, with monitoring being conducted either optically using photographic or video apparatus or by spectroscopic methods.
 11. Use according to claim 10, characterized in that the monitoring takes place by the reflective mode, i.e. the beam passing through the observation window in the container for monitoring the process is reflected at the wall of the process container opposite the window and exits through the same window, and is picked up for analytical evaluation.
 12. Use according to claim 10, characterized in that at least two observation windows according to one of claims 7 to 9 are arranged in series in the container and monitoring takes place by transmission. 